Generally, the required torque during the work implemented by a power tool, such as boring or screwing, will be periodically changed usually along with the differences of the working schedule and the processed material. For example, at the beginning of a working period, it usually needs low torque, while at the end of this period, it needs higher torque. In the prior art, the regulation of the speed regulating device is usually furnished by manual operation, which causes the operator to have to pay attention to the working state and the load changing condition of the tool all the times, and determine when the speed regulating device needs to be adjusted. Such manual operation requires the operator to possess the high level of the operation technique so that it is inconvenient to improve the efficiency